The existence of quark stars has not been confirmed, either theoretically or astronomically. The equation of state of quark matter is uncertain, as is the transition point between neutron-degenerate matter and quark matter. Theoretical uncertainties have precluded making predictions from first principles. Experimentally, the behaviour of quark matter is currently being actively studied with particle colliders, although this can only produce hot quark-gluon plasma blobs the size of an atomic nucleus, and they decay immediately after formation. There are no known artificial methods to produce or store cold quark matter as found in quark stars.

Ordinary quark matter consisting of up and down quarks (also referred to as u and d quarks) has a very high Fermi energy compared to ordinary atomic matter and is only stable under extreme temperatures and/or pressures. This suggests that only quark stars comprising neutron stars with a quark matter core will be stable, while quark stars consisting entirely of ordinary quark matter will be highly unstable and dissolve spontaneously.[2][3]

It has been shown that the high Fermi energy making ordinary quark matter unstable at low temperatures and pressures can be lowered substantially by the transformation of a sufficient number of u and d quarks into strange quarks, as strange quarks are, relatively speaking, a very heavy type of quark particle.[2] This kind of quark matter is known specifically as strange quark matter and it is speculated and subject to current scientific investigation whether it might in fact be stable under the conditions of interstellar space (i.e. near zero external pressure and temperature). If this is the case (known as the Bodmer–Witten assumption), quark stars made entirely of quark matter would be stable if they quickly transform into strange quark matter.[4]

Quark stars made of strange quark matter are known as strange stars, and they form a subgroup under the quark star category.[4]

Strange stars might exist without regard of the Bodmer-Witten assumption of stability at near-zero temperatures and pressures, as strange quark matter might form and remain stable at the core of neutron stars, in the same way as ordinary quark matter could.[1] Such strange stars will naturally have a crust layer of neutron star material. The depth of the crust layer will depend on the physical conditions and circumstances of the entire star and on the properties of strange quark matter in general. Stars partially made up of quark matter (including strange quark matter) are also referred to as hybrid stars.[5][6]

Theoretical investigations have revealed that quark stars might not only be produced from neutron stars and powerful supernovas, but they could also be created in the early cosmic phase separations following the Big Bang.[2] If these primordial quark stars transform into strange quark matter before the external temperature and pressure conditions of the early Universe makes them unstable, they might turn out stable, if the Bodmer–Witten assumption holds true. Such primordial strange stars could survive to this day.[2]

If the conversion of neutron-degenerate matter to (strange) quark matter is total, a quark star can to some extent be imagined as a single gigantic hadron. But this "hadron" will be bound by gravity, rather than the strong force that binds ordinary hadrons.

Recent theoretical research has found mechanisms by which quark stars with "strange quark nuggets" may decrease the objects' electric fields and densities from previous theoretical expectations, causing such stars to appear very much like—nearly indistinguishable from—neutron stars. However, the investigating team of Prashanth Jaikumar, Sanjay Reddy and Andrew W. Steiner made some fundamental assumptions, that led to uncertainties in their results large enough that the case is not finally settled. More research, both observational and theoretical, remains to be done on strange stars in the future.[7]

Other theoretical work[8] contends that, "A sharp interface between quark matter and the vacuum would have very different properties from the surface of a neutron star"; and, addressing key parameters like surface tension and electrical forces that were neglected in the original study, the results show that as long as the surface tension is below a low critical value, the large strangelets are indeed unstable to fragmentation and strange stars naturally come with complex strangelet crusts, analogous to those of neutron stars.

Jaffe 1977 suggested the H dibaryon, a six-quark state with equal numbers of up-, down-, and strange quarks (represented as uuddss or udsuds).

Bound multi-quark systems with heavy quarks (QQqq).

In 1987, a pentaquark state was first proposed with a charm anti-quark (qqqsc).

Pentaquark state with an antistrange quark and four light quarks consisting of up- and down-quarks only (qqqqs).

Light pentaquarks are grouped within an antidecuplet, the lightest candidate, Ө+.

This can also be described by the diquark model of Jaffe and Wilczek (QCD).

Ө++ and antiparticle Ө−−.

Doubly strange pentaquark (ssddu), member of the light pentaquark antidecuplet.

Charmed pentaquark Өc(3100) (uuddc) state was detected by the H1 collaboration.[9]

Tetra quark particles might form inside neutron stars and under other extreme conditions. In 2008, 2013 and 2014 the tetra quark particle of Z(4430), was discovered and investigated in laboratories on Earth.[10]

Statistically, the probability of a neutron star being a quark star is low,[citation needed] so in the Milky Way Galaxy there will only be a small population of quark stars. If it is correct however, that overdense neutron stars turn into quark stars, that makes the possible number of quark stars higher than was originally thought, as we would be looking for the wrong type of star.

Quark stars and strange stars are entirely hypothetical as of 2011[update], but observations released by the Chandra X-ray Observatory on April 10, 2002 detected two candidates, designated RX J1856.5-3754 and 3C58, which had previously been thought to be neutron stars. Based on the known laws of physics, the former appeared much smaller and the latter much colder than it should be, suggesting that they are composed of material denser than neutron-degenerate matter. However, these observations are met with skepticism by researchers who say the results were not conclusive;[11] and since the late 2000s, the possibility that RX J1856 is a quark star has been excluded (see RX J1856.5-3754).

It was reported in 2008 that observations of supernovae SN2006gy, SN2005gj and SN2005ap also suggest the existence of quark stars.[14] It has been suggested that the collapsed core of supernova SN1987A may be a quark star.[15][16]